In many applications where a large spring force is needed, such as in clutches employed in agricultural vehicles, coil springs have limited use because of the space that they need. Furthermore, coil springs present a variable spring rate, the force required to deflect the springs increasing as the deflection increases.
Frustro-conical spring discs or so-called Belleville springs are not only capable of having very high strength for the space occupied, but can be constructed and preloaded to have a constant spring rate. When Belleville springs are assembled in coaxial stacks, in peripheral engagement, the inner and outer circumferences of the springs need to be correctly aligned during installation to maintain concentricity. Incorrect centering can result in hysterysis which renders the spring system faulty or non-uniform in its operation.
Hysterysis can be understood from a consideration of a graph plotting spring force against displacement as the stack of springs is first compressed then allowed to expand. Ideally, both during the compression and the expansion parts of this cycle, the spring force at any given displacement should be the same and the graph should simply track up then down the same line. In practice, however, the graph follows a closed loop, not a single line, and the greater the hysterysis, the greater the area enclosed by the loop.
Hysterysis is caused by friction as the edges of the springs rub and slide against one another instead of the lines of contact acting as pivots. To minimize hysterysis losses and wear, the springs in a stack need therefore to be correctly aligned.
U.S. Pat. No. 1,826,597, U.S. Pat. No. 2,020,927, U.S. Pat. No. 3,010,713 and U.S. Pat. No. 3,873,079 use different arrangements of spacer rings between the individual springs in a stack to retain them in concentric relationship. There is however no need for this complication because, once correctly installed, the springs remain permanently under compression and do not tend to move relative to one another. It is thus only during initial installation that special care is required to align them with one another and such alignment has previously been carried out by eye. Even if the individual springs are correctly placed one above the other, it is not certain that they will remain aligned when they are compressed during their installation about a cylindrical shaft or within a cylindrical bore.